Lithographic printing presses use a so-called printing master such as a printing plate which is mounted on a cylinder of the printing press. The master carries a lithographic image on its surface and a print is obtained by applying ink to said image and then transferring the ink from the master onto a receiver material, which is typically paper. In conventional, so-called “wet” lithographic printing, ink as well as an aqueous fountain solution (also called dampening liquid) are supplied to the lithographic image which consists of oleophilic (or hydrophobic, i.e. ink-accepting, water-repelling) areas as well as hydrophilic (or oleophobic, i.e. water-accepting, ink-repelling) areas. In so-called driographic printing, the lithographic image consists of ink-accepting and ink-abhesive (ink-repelling) areas and during driographic printing, only ink is supplied to the master.
Printing masters are generally obtained by the image-wise exposure and processing of an imaging material called plate precursor. In addition to the well-known photosensitive, so-called pre-sensitized plate precursors, which are suitable for UV contact exposure through a film mask, also heat-sensitive printing plate precursors have become very popular in the late 1990s. Such thermal materials offer the advantage of daylight stability and are especially used in the so-called computer-to-plate method wherein the plate precursor is directly exposed, i.e. without the use of a film mask. The material is exposed to heat or to infrared light and the generated heat triggers a (physico-)chemical process, such as ablation, polymerization, insolubilization by crosslinking of a polymer, heat-induced solubilization or particle coagulation of a thermoplastic polymer latex.
The most popular thermal plates form an image by a heat-induced solubility difference in an alkaline developer between exposed and non-exposed areas of the coating. The coating typically comprises an oleophilic binder, e.g. a phenolic resin, of which the rate of dissolution in the developer is either reduced (negative working) or increased (positive working) by the image-wise exposure. During processing, the solubility differential leads to the removal of the non-image (non-printing) areas of the coating, thereby revealing the hydrophilic support, while the image (printing) areas of the coating remain on the support. Typical examples of such plates are described in e.g. EP-A 625728, 823327, 825927, 864420, 894622 and 901902. Negative working embodiments of such thermal materials often require a pre-heat step between exposure and development as described in e.g. EP-625,728.
In the graphic arts industry, there is an evolution towards the use of recycled paper and more abrasive inks, fountain solutions and/or plate cleaners. These harsh printing conditions, especially occurring on web presses, not only impose more stringent demands on the chemical resistance of the printing plates towards pressroom chemicals and inks but also reduce their press life. To improve the chemical resistance and/or press life of positive-working plates based on oleophilic resins, often a heat-treatment is carried out after the exposure and development steps. However, this heat-treatment, also known as post-baking, is both energy and time consuming. Other solutions to these issues have been provided in the art by optimizing the coatings for example by selection of specific alkaline soluble resins—e.g. by chemical modification—and/or by providing double layer coatings. Such coatings typically include a first layer comprising a highly solvent resistant alkaline soluble resin and a second layer on top of this first layer comprising a phenolic resin for image formation. In addition, positive-working printing plate precursors based on a solubility difference may suffer from an insufficient development latitude, i.e. the dissolution of the exposed areas in the developer is not completely finished before the unexposed areas also start dissolving in the developer. This often results in insufficient clean-out leading to toning (ink-acceptance at the non-image areas), a loss of coating (small image details) at the image areas, a reduced press life and/or a reduced chemical resistance of the printing plate. An optimized lithographic latitude requires an excellent clean-out of the non-image parts upon processing while maintaining an excellent run length performance and image quality. Both run length and clean-out performance are determined by the interaction between the heat-sensitive coating and the substrate. An optimal run length requires adhesion between the substrate and the heat sensitive layer in the image parts, while clean-out requires minimal interaction in the non-image parts upon processing. As a result, maximizing the clean-out performance often results in image attack and reduced image quality, such as an increased undercutting in double layer plates. Therefore, there is still a need for new coatings satisfying both runlength and clean-out requirements at the same time.
EP 1 826 001 and EP 2 159 049 disclose a heat-sensitive, positive-working lithographic printing plate precursor comprising on a support having a hydrophilic surface or which is provided with a hydrophilic layer a heat-sensitive coating comprising an IR absorbing agent, a phenolic resin and a polymer including a monomeric unit having a sulfonamide group.
WO 02/053627, US 04/0023155 and US 02/160299 disclose a positive working lithographic printing plate precursor comprising a thermally sensitive supramolecular polymer which may include a base soluble group such as a phenolic group and/or a carboxylic acid group.
JP2009086326 and JP2009014779 disclose a lithographic printing plate including an intermediate layer between the aluminium support and the image forming layers including a specified polymer including a monomeric unit having an aromatic group containing hydroxyl and carboxyl substituents.